There are many types of circuits for powering a load. One type of circuit for powering a load is an inverter circuit. An inverter circuit receives a direct current (DC) signal, from a rectifier for example, and outputs an alternating current (AC) signal. The AC output can be coupled to a load, such as a fluorescent lamp, or to a rectifier so as to form a DC-DC converter.
There are different types of inverter circuits which can have a variety of configurations. One type of inverter circuit, known as a half-bridge inverter circuit, includes first and second switching elements, such as transistors, coupled in a half-bridge configuration. Another type of inverter circuit referred to as a full-bridge inverter circuit includes four switching elements coupled in a full-bridge configuration. Half-bridge and full-bridge inverter circuits are typically driven at a characteristic resonant frequency determined by the impedance values of the various circuit elements.
For a resonant inverter, the current to the load periodically reverses direction. That is, the current flows through the load in a first direction for a first half of a resonant cycle and reverses direction after a period of time determined by the resonant frequency. The current then flows through the load in a second, opposite direction during a second half of the resonant cycle. For a half bridge inverter, the first switching element is conductive for the first half of the resonant cycle and the second switching element is conductive for the second half of the resonant cycle. And for a full bridge type configuration, first and second switching elements conduct for half of the resonant cycle and third and fourth switching elements conduct for the second half of the cycle.
To operate the circuit at or near resonance, the conduction states of the switching elements need to be controlled. Generally, each switching element is controlled by a respective control circuit which biases each switching element to a conductive state for about half of the resonant cycle and to a non-conductive state for the second half of the cycle. One type of control circuit includes an inductive bias element inductively coupled to a resonant inductive element through which current to the load flows. The bias element applies a potential to the switching element, such as to the base terminal of a bipolar junction transistor, for biasing the switching element to a conduction state that corresponds to a direction of current flow through the load. The switching element control circuits, as well as the switching elements themselves, can require significant space on a circuit board. It will be appreciated by one of ordinary skill in the art that, in general, space on a circuit board is at a premium.
A further drawback associated with certain ballasts occurs when a lamp fails to light. Due to the resonant nature of the inverter circuit, relatively high voltage levels can be generated when a lamp fails to light. Such high voltage levels can have a negative impact on circuit performance and may cause damage to the circuit components. In addition, a ballast coupled to multiple lamps may not provide satisfactory operation of the other lamps when one lamp fails to light. Further, ballast circuits are not generally modular so that a failure of a component associated with energizing one lamp or one set of lamps necessitates replacement of the entire ballast.
It would be desirable to provide a ballast circuit which regulates lamp current so as to prevent excessive voltage levels from occurring when a lamp fails to light. It would also be desirable to provide a ballast circuit that can energize different types of lamps. It would further be desirable to provide a ballast for independently energizing a plurality of lamps which has a plurality of replaceable modules to allow one or more faulty component associated with a particular lamp to be replaced.